EPR spectra of two series of electrochemically generated macrocyclic mixed-valence copper complex species [Cu(II)Cu(I)L] + have been investigated. Complexes belonging to series 1 [Cu 2L](ClO 4) 2 are derived from 6,12,18,24-substituted (H,H,H,H, ( a); Me,Me,Me,Me, ( b); Pr,Pr,Pr,Pr ( c); Ph,Ph,Ph,Ph ( d); H,H,Me,Me ( e)) macrocycle 7,11,19,23-dimetheno-9,21-dimethyl-1,5,13,17-tetra-azacyclotetracosa-5,7, 9,12,17,19,21,14-octaene-25,26-diol (H 2L). Complexes belonging to series 3, [Cu 2L](ClO 4) 2, are related to those of series 1 by replacement of two of the CHN linkages by CH 2NH groups of the segment containing two azomethine linkages having the 6,12-substituents H,H ( a); Me,Me ( b); Me,Ph ( c). In the mixed-valence species derived from 1b–1e and 3a–3c in MeCN or CH 2Cl 2 solutions the single unpaired electron is localized on one of the metal centres at room temperature on the EPR time scale. Only the species derived from 1a gives a seven-line EPR spectrum at room temperature indicating that the unpaired electron is delocalized over both metal centers. Variable temperature magnetic measurements on complexes 1b and 1c indicate strong antiferromagnetic exchange (−2 J = 835 ± 5 cm −1 and 806 ± 9 cm −1, respectively) in both cases.